Can you tell which ducky was swimming with a conventional outboard motor? When testing the pollution released from boat engines, the California Air Resources Board ran different engines in enclosed tanks. Duckies swimming in the tank with a new 4-stroke engine came out looking like the one on the left, while those sentenced to swim with the conventional 2-stroke engine, like the ducky on the right, came out the worse for wear. (Thanks to Sonia Hamel of the Office of Commonwealth Development for the loan of the duckies!)

Conventional 2-Stroke Outboard Engines
Conventional motorboat and Personal Watercraft (PWC) engines use a carbureted 2-stroke outboard system, available from 2 to 300 horsepower (hp). These engines do not completely burn fuel, instead directly releasing as much as 20 to 30 percent into the air or water. The reason is that both the intake and exhaust ports of the engine are open at the same time, allowing the fuel to pass directly through. These engines not only pollute the air and water, they waste $2 to $3 dollars for every $10 boaters spend on gas.

Direct Fuel Injection Engines
Newer 2-stroke technology has lead to higher fuel efficiency and lower hydrocarbon emissions levels. These Direct Fuel Injection (DFI) engines spray fuel into the cylinder when the piston covers the intake and exhaust ports, preventing this direct release of unburned fuel. They are available in the 40 to 250 hp range.

4-Stroke Engines
Although DFI engines are good, 4-strokes are the cleanest outboard engines currently available up to 250 horsepower. Intake and exhaust valves are never open at the same time, preventing the release of unburned fuel from the combustion chamber. This technology leads to excellent fuel efficiency. The 4-stroke engines weigh about 25 percent more than conventional engines.

The Benefits
When compared to conventional 2-stroke engines, 4-stroke and DFI engines:

• Improve fuel efficiency (25-45 percent).
• Reduce hydrocarbon emissions by 75-90 percent.
• Reduce smoke smell.
• Are quieter.
• Improve drivability with smoother combustion.
• Start more easily under both hot and cold conditions.
• Idle better at low speed (lower vibration/noise).

In addition, DFIs decrease oil consumption by 50 percent, while 4-strokes run on straight gasoline (rather than an oil/gas mix), so no oil is consumed during combustion, reducing oil released to the air and water. (However, like an automobile engine, oil in 4-stroke engines must be changed as part of routine maintenance.) And even though these new engines cost about 20 percent more than conventional 2-strokes, they quickly save the boater money in lower fuel and oil costs.

For More Information . . .
For more on clean boating programs, e-mail Robin Lacey at robin.lacey@state.ma.us.

Adapted in May 2003 from an article in the spring 2001 edition of Coastlines, the CZM newsletter. Author: Anne Donovan, CZM.